The radio spectrum is a finite resource expected to accommodate ever increasing numbers of communication nodes whether in a commercial environment or in a military environment. Tactical military and commercial applications require self-organizing, wireless networks that can operate in dynamic environments and provide peer-to-peer communications. For multiplexed communication systems, the communication system typically consists of many communication nodes that require active service over a single communication channel. As a result, a variety of multiple access schemes have been devised that allow multiple users to share the same communication channel. For example, communication systems have been developed to provide communication between many communication nodes for brief intervals of time using the same communication channel. Such a multiple access scheme is known as Time Division Multiple Access (TDMA) protocol. A communication system that employs TDMA is referred to herein as a TDMA system or a TDMA network.
A TDMA communication system can be used in conjunction with other types of multiple access systems including frequency division multiple access and code division multiple access systems. For example, frequency hopping using a frequency synthesizer to “hop” the signal from one frequency to the next can be used in a TDMA communication system to improve performance characteristics including providing a low probability of intercept and reducing the effects of co-channel interference, multi-path, and fading. The communication nodes are synchronized to follow a frequency hopping code thereby ensuring that the communication nodes are on the same frequency at the same time. The hopping pattern may be assigned to each node in its identification code. As a result, it is difficult for a communication node that does not know the frequency hopping code to synchronize with the other communication nodes.
An advantageous characteristic of a TDMA system is the ability of neighboring nodes to transmit without interference. In a TDMA system, each carrier frequency is divided into repeating frames. The frames are subdivided into a plurality of time slots. A communication node within the TDMA system is assigned one or more time slots for transmitting and/or for receiving a communication signal. Each communication node is assigned particular time slots in a continuum of recurrent frames for transmission of its bursts and for reception of the bursts of other nodes. The time slots are designed to be non-overlapping when the various nodes' signals arrive at the receiver, and thus, include a time delay for a signal propagation time. A base radio may manage all of its assigned communication nodes by keeping the nodes synchronized and allowing each node to communicate at a known, or deterministic, time. Alternatively, the communication nodes may use a self-organizing system to allocate time slots and to maintain synchronization with each other. For example, the Unifying Slot Assignment Protocol (USAP), which is disclosed in U.S. Pat. No. 5,719,868, provides a protocol for maintaining such a self-organizing communication system.
Mobile, multi-hop, broadcast packet radio networks provide rapid and convenient deployment, self organization, mobility, and survivability. In this type of network, a transmission from one node is broadcast to all nodes in its “neighborhood”. With the growth in the number of communication nodes, the need to quickly form and/or merge formed networks as nodes move into a neighborhood has increased. Ultra-high frequency (UHF) systems generally have a neighborhood defined by nodes within line of sight of the transmitting node. For illustration, FIG. 1 depicts an overlapping neighborhood system 10 that includes a first neighborhood 18, a second neighborhood 20, and a third neighborhood 22. The first neighborhood 21 includes nodes 1-5 and f that are within line of sight of node 3. The second neighborhood 22 includes nodes 2, 3, 5, a, c, d, and f that are within line of sight of node f. The third neighborhood 23 includes nodes a, b, c, d, e, f, and g that are within line of sight of node c. A node in the first neighborhood 21 can communicate with a node in the third neighborhood 23 using multiple “hops” from a node in the second neighborhood 22. All of the nodes in the overlapping neighborhood system 10 or even within a neighborhood may not be in communication with each other. For example, two networks encompass the communication nodes of system 10. A first network 12 includes nodes 1-5 that are in communication with each other, and a second network 14 includes nodes a, b, c, d, e, f, and g that are in communication with each other. Multiple hops may be required, for example, to provide communication between node f and node e. The second neighborhood 20, however, includes nodes 2, 3, and 5 that are not currently in communication with node f as indicated by the broken lines. The ability to first form the networks 12 and 14 and then to merge network 12 with network 14 in a time and a power efficient manner is an important function of a TDMA system.
To avoid interference between the transmissions from different communication nodes, a TDMA system requires good clock synchronization between the communication nodes. Timing acquisition is a process for synchronizing the receiver's clock with the transmitter's clock so that the receiver can determine the boundary between two transmitted symbols. In general, timing acquisition is performed by sending a preamble before information bits in a TDMA frame. Additionally, each communication signal transmitted in a time slot includes a predefined synchronization word. The synchronization word must be matched in order to validate the communication that follows. If no match is achieved, the information can not be processed from the communication signal. By identifying the location of the synchronization word in a received signal, a node maintains synchronization with the other nodes in the network. This scheme requires a reasonably accurate clock timing recovery before frame synchronization (or any form of communication) can take place. Any false or missed detection of the synchronization pattern results in a loss of the information in the data frame. This poses a particular problem in a TDMA system operating in a dynamic environment in which nodes move in and out of neighborhoods because the new node must synchronize with the TDMA system before communication between the new node and the TDMA system can be accomplished.
For example, to establish synchronization with a new node in a neighborhood comprising an active network such as the node f relative to network 12, a frame synchronization pattern is periodically inserted into the data stream by a transmitter within the first network 12. In an example protocol, once every few seconds a network synchronization time slot is allocated to transmit the frame synchronization pattern. A communication node within the first network 12 may pseudo randomly transmit the network synchronization information. The remaining communication nodes in the first network 12 listen. The node f may similarly transmit synchronization information for the second network 14. Based on statistical theory, the first network 12 and the second network 14 can be merged within an expected time delay assuming that both the first network 12 and the second network 14 are using the same TDMA protocol, for example, based on one of the nodes a, c, d, or f receiving the network synchronization information from a node (for example, nodes 2, 3, or 5) included in the first network 12 or a node included in the first network 12 receiving the synchronization information from one of the nodes a, c, d, or f.
After a communication node is powered on, the node uses 100% of its receiver time to search for other nodes with which to form an ad hoc network. The communication node is also transmitting a status message in a pseudo random manner for reception by other nodes that may be listening. With some time delay, the communication node finds another node's status transmission or vice versa, and the two nodes form a “sub-network”. Information begins to flow between the two communication nodes using most of the available communication resources in order to maintain network efficiency. Currently, only a small percentage of the communication resources are reserved to allow each node to “look around” and identify other nodes within the neighborhood that may be transmitting a status message. As a result, such a search-and-make-a-network algorithm forms many sub-networks. The initial sub-network may form relatively quickly, but the statistical opportunities to locate and to communicate with other nodes or sub-networks drops significantly based on the reduction in allocated resources resulting in significant time delays before other nodes or sub-networks in the same neighborhood join the sub-network or form a common network.
A common and accurate representation of time is assumed in determining the likelihood of successful synchronization and the expected time delay associated with forming a network such as the first network 12 and with merging, for example, the first network 12 with the second network 14. If, however, the clock of the second network 14 is not synchronized with sufficient accuracy to the clock of the first network 12, significant time delays (possibly on the order of hours or even days) can result before synchronization between the first network 12 and the second network 14 is completed. Significant delays may result due to clock inaccuracies of as little as 100 μs. What is needed, therefore, is a system and a method that improve the formation of a network that includes the communication nodes within a neighborhood. What is further needed is a system and a method that improve the merging of formed networks into a single network. What is further needed is a system and a method that reduce the waste of communication resources.